The sum of a and b will be the coefficient of the x term of the trinomial. This can be seen below.
(x + a)(x + b)
= x² + bx + ax + ab
= x² + (a+b)x + ab
x is being multiplied by both a and b individually and the products are being added, so sum of a and b appears as the coefficient of x term.
Thus the answer to this question is option C